The invention relates to a pellet mill for the production of extruded pellets, in particular animal food pellets from a flour product. The pellet mill may form part of an assembly comprising a feeding section to which the flour product is supplied, a conditioning section for blending the flour product with additives, such as molasses and/or for heating and moistening the mixture, and a press section or pellet mill for extruding the prepared flour product in the form of hard pellets of, f.i. cylindrical cross section. An assembly of this type is disclosed in U.S. Pat. No. 3,828,661.
Conventional pellet mills of the present type usually comprise a rotary annular die having a great number of radial die openings and enclosing an interior die space which is closed on one side and open on the opposite feed side. At its closed side the die is supported on one end of a horizontally mounted hollow drive shaft. A number of extrusion rollers, f.i. two such rollers are rotatably mounted in a common roller frame arranged in the interior of the die, the extrusion rollers cooperating with the inner cylindrical surface of the annular die in pressing the flour product fed to the die radially outwardly through the die openings. The roller frame is secured on one end of a second shaft extending through and rotatably mounted in the hollow drive shaft, shear pin means or the like connecting the opposite end of this second shaft to the frame of the mill for holding the second shaft and thus the roller frame stationary during normal operation of the mill. Feed means are arranged opposite the open side of the rotary annular die for feeding the flour product to be pressed to the interior of the die.
During the operation of a pellet mill of the above-described type the feed means should preferably feed the flour product to the die forwardly of the extrusion rollers (as seen in the direction of rotation of the die) allowing these rollers to press the material through the die openings whereby bar-shaped pellets are extruded which are cut to length by cutters engaging the cylindrical outer surface of the die. For a proper and efficient operation of the pellet mill it is obviously of importance that the material to be pressed is not only fed in equal portions to the several extrusion rollers but is also distributed evenly over the axial length of each roller and thus also over the effective width of the annular die in order to make full use of the capacity of the pellet mill and to avoid uneven wear of the rollers and of the die. However, for obtaining such an even distribution it is generally not allowable to use feed means which extend from outside the die into the die interior. The reason for this is that the extrusion rollers and the die must be protected against possible damage caused by overloading or by the occurence of foreign hard matter such as a piece of iron or stone, in the supplied flour product. If such a foreign hard body is clamped between a roller and the die and consequently the driven die exerts a rotational force on the roller frame, the shear pins normally holding the shaft of the roller frame stationary will break whereby the roller frame can rotate together with the die before the hard body can cause rupture of the die or other damage. Rotation of the central shaft supporting the roller frame causes the drive motor of the mill to be switched off. Since thus the roller frame must be free to rotate in cases of emergency, there should be no stationary parts, such as feed members, extending into the interior of the die. For this reason, in the conventional pellet mills of the present type the flour product is supplied to the open front side of the die from which the material must spread out across the die under centrigual action and sometimes with the help of suitable blades attached to the rotating die or by means of scraper elements mounted on the stationary central shaft. Generally, the material is fed at a single location at the lower side of the die independently of the number of the extrusion rollers and it will be clear that in this manner the above-mentioned even distribution of the supplied material over the several rollers and over the width of the die is rather illusory. A further consequence of this situation is that the die and the extrusion rollers can only have a limited axial dimension. Furthermore in these conventional feeding systems the distribution of the material in the die depends to a great extent on the properties of the flour product, such as the bulk density, the particle size and the fluidity thereof while also the rotational speed of the die has its influence.